1. Field of the Invention
The present invention relates to a method for producing a golf ball with a structure including a spherical elastic inner layer having a large number of projections on the outer surface, and the periphery thereof covered with a thermoplastic resin outer layer.
2. Description of the Related Art
As golf balls, a rubber thread type produced by covering a core formed by winding a rubber thread spherically with a balata cover, and a solid type produced by covering a hard rubber spherical core having a high resilience with a resin cover having excellent tear resistance and wear resistance are commonly known. Among them, since the latter one is superior in terms of the flying distance, this type is prevailing among golfers.
However, since the solid golf ball still has points to be improved, such as a relatively hard hit feeling and relative shortage of spinning when hit by a short iron with a large loft, the following improved examples are known.
The first one is a structure with one or two relatively thin intermediate layers having a hardness lower than that of a cover, disposed between a core and the cover. The second one is a structure with an intermediate layer having a large number of small projections provided on the outer surface, disposed between a core and a cover. Also in this case, in general, the intermediate layer is formed of a resin layer having a hardness lower than that of the cover. Further, the height of the projections is set to be slightly lower than the thickness of the cover in most cases. Accordingly, since the cover material can be disposed among the projections of the intermediate layer, a third layer having a property mixed with those of the cover and the intermediate layer can substantially exist in the small projection portions in the radial direction of the golf ball. Moreover, it is also advantageous in that the physical bond between the cover and the intermediate layer can be reinforced.
Although a structure wherein a plurality of layers with different physical properties forming a golf ball are bonded via a large number of small projections is advantageous as mentioned above, it is extremely difficult to produce as mentioned below.
In forming the resin intermediate layer or the resin cover around the rubber core, or in forming the resin cover around the resin intermediate layer, a two-piece mold having a spherical cavity in the inside is used. More specifically, the shaping operation is executed using a mold comprising an upper mold and a lower mold with the parting surface provided at a position corresponding to an equator of a spherical cavity, with the pole sections of the upper mold and the lower mold each provided with a plurality of columnar support pins elongating toward the parting surface reciprocally movably.
That is, a core is set at the center of the cavity by inserting the core or the core covered with an intermediate layer at the center portion of the cavity in the mold, closing the upper mold and the lower mold, and introducing the support pins provided in the upper mold and the lower mold into the cavity from above and below. In this case, if the outer surface of the core is smooth, it is easy and certain to hold the core with the support pins, and thus it is set at the center position of the cavity accurately. However, in the case of a core or an intermediate-layer-covered core having a large number of small projections on the outer surface, it cannot be held with the support pins stably, and thus it is liable to be set at a position off the cavity center. If a cover material is injected with the core set at the eccentric position of the cavity, it is apparent that a golf ball lacking the uniformity in terms of the structure and the performance, setting aside the external appearance, is shaped.
In order to solve the above-mentioned problem, an object of the invention is to provide a method for producing a golf ball having the excellent uniformity using a core having a large number of small projections on the outer surface, or an inner layer comprising an intermediate-layer-covered core.
In order to achieve the above object, according to the invention, there is provided a method for producing a golf ball including a spherical elastic inner layer having a large number of projections with a round shape or a polygonal shape when viewed two-dimensionally on the outer surface with a predetermined space, and a thermoplastic resin outer layer covering the inner layer, comprising the steps of: providing a mold comprising an upper mold and a lower mold with a spherical cavity formed in the inside and a parting surface provided at a position corresponding to an equator of the spherical cavity, with pole sections of the upper mold and the lower mold each provided with a plurality of columnar support pins elongating into the cavity reciprocally movably; supporting the inner layer at a cavity center portion of the mold by the support pins from above and below at the outer surface space position; and injecting an external layer resin into a space between the inner layer and the cavity wall surface so as to integrally form the inner and outer layers.
As an embodiment of the invention, a structure wherein the inner layer is a rubber core having a large number of projections on the surface, and the outer layer is a resin cover provided with dimples on the surface can be provided.
As another embodiment, a structure wherein the inner layer is a rubber core having a large number of projections on the surface, and the outer layer includes a resin intermediate layer provided between the cover and the rubber core can be provided.
As still another embodiment, a structure wherein the inner layer is a resin intermediate layer disposed on a rubber core, and the outer layer includes a resin cover can be provided.
It is preferable that the projections have a tapered shape having a 0.8 to 3 mm base portion diameter and a 0.3 to 5 mm height.
Moreover, it is preferable that 50 to 500 pieces of the projections are provided substantially uniformly on the outer surface of the inner layer.
Furthermore, it is preferable that at least 3 pieces (3 to 8 pieces) of the inner layer support pins are provided inside a 12.5 mm radius circle centering on each pole of the upper mold and the lower mold with an equal interval, and it is further preferable that 4 to 5 pieces are provided with an equal interval. Moreover, the width or the diameter of the support pins is preferably 0.5 to 5 mm.